A fuel cell vehicle power system often requires a high-power DC / DC converter to connect the fuel cell to the power drive system and energy storage system to achieve power regulation, and the boost ratio of the high-power DC / DC converter is generally not large, and usually does not require isolation. Comprehensive analysis of the existing DC / DC converter structure, non-isolated Boost converter has a simple structure, high efficiency, and continuous input current, which is very suitable for fuel cell vehicle power regulation.
However, in the traditional high-power hard-switching converter, there is a current tailing phenomenon when the IGBT is turned off, and the turn-off loss is very serious. Most of the traditional soft switches use the principle of resonance to make the current (voltage) in the power tube change according to the law of sine or quasi-sine. When the current naturally crosses zero or the voltage is zero, the power tube is turned on or off, thereby reducing the switching loss of the power tube.
A new type of soft-switching Boost converter is proposed to solve the problem of the switching loss of high-power IGBTs. Compared with other soft-switched Boost converters, the converter has a simple circuit, small size, light weight, and high efficiency. It can work reliably in the full load range and is very suitable for fuel cell vehicles.
2.Capacitor buffered soft-switching Boost converter
2.1 Circuit structure
For high-power Boost converters using IGBTs, the turn-off loss of the power tube is usually much larger than its turn-on loss. The use of a SiC diode with almost zero reverse recovery current can effectively reduce the turn-on loss of the power tube. Capacitive buffered soft switching is mainly used to solve the switching-off loss of the main power tube V1 in the Boost converter. During the V1 turn-off process, the capacitor buffer circuit slows down the voltage rise rate of V1 and reduces the current tailing when V1 turns off, thereby effectively reducing the V1 turn-off loss. Figure 1 shows a capacitor buffered soft-switched Boost converter. Among them, VD1 is the main diode, L1 is the main boost inductor, and C1 is the output filter capacitor. The capacitor buffer circuit is composed of auxiliary switching tubes V2 and V3, auxiliary diodes VD2 and VD3, and a buffer capacitor C2.
Figure 2 shows the main operating waveforms of a capacitor buffered soft-switched Boost converter
2.2, soft switch implementation conditions
It can be seen that the implementation conditions of this soft switch are very simple, as long as V2 and V3 are turned on before V1 is turned off, and they can be turned off before V1 is turned on again. V2 and V3 use a constant duty cycle to achieve C2 charging (V3 on) and discharge (V2 on). The off time of V1 is synchronized with the on time of V2 and V3. Its proportion can be adjusted by adjusting the on time of V1 Air ratio.